2018年最致命的地震發生時,灌溉稻田的系統造成山崩更加嚴重
新加坡南洋理工大學科學家主持的國際研究顯示,印尼蘇拉威西島的巴路市在2018年地震時引發的山崩,因為灌溉而變得更加慘重。
大量灌溉的稻田人為提升地下水面至接近地表的地方,造成2018年印尼巴路市由地震引發的山崩變得更加嚴重。圖片來源:Gilles
Brocard/University of Sydney
2018年9月28日,一場規模7.5的地震襲擊了印尼這座城市,超過4300人因而罹難,使其成為當年全世界死亡人數最多的地震。
新加坡南洋理工大學的新加坡地球觀測站與亞洲環境學院的研究人員,聯合印尼、美國、英國、中國、澳洲的學術機構,在《自然―地球科學》(Nature
Geoscience)發表的研究中指出印尼巴路谷中的山崩是由大範圍的液化現象造成,發生在為了種植稻米而大量灌溉的地區。
巴路谷中建有一條已有百餘年歷史的水道系統,目的是將足夠的水分運來種植稻米。這項人為因素使得地下水位升到幾近地表的位置,造成「土壤液化」的機率提高。當地震波造成地表劇烈晃動,使得底下的沉積物變成像液體一樣,這種狀況即為土壤液化。
猶如液體般的沉積物加上河谷本身的坡度,使得水、山崩、土石的流動範圍變得十分廣大,淹沒許多村莊而擴大災情。
這些致命的因素混合起來使這次地震成為2006年的日惹地震之後,印尼死傷人數最多的地震。
南洋理工大學新加坡地球觀測站的Kyle
Bradley是這項研究的計畫主持人,他說:「這場事件對於任何有活斷層經過並且有在進行灌溉的地區來說是個警訊。」
Kyle Bradley也是南洋理工大學亞洲環境學院的講師,他說:「我們需要更加認識並注意和土壤液化有關的山崩;此外,也要密切關注灌溉造成地下水面人為升高的地區。」
研究強調出東南亞各國應該立即審視活斷層沿線稻作活動相當密集的地區。
Bradley博士表示:「東南亞各國的發展速度通常比大地震的再現時間,也就是兩場大地震之間的平均間隔還長,這點令人相當擔憂。其他有在灌溉的類似地區,很多都還不曾受到極為強烈的地面晃動考驗,其中某些也有發生重大災難的可能性。」
研究今昔的衛星資料
南洋理工大學的研究人員分析了地震前後拍攝的衛星影像,藉此辨識出受到山崩影響的地區。他們發現有在灌溉的稻田受到的影響十分劇烈,但是其他種有樹木的地方大部分都還是處於穩定狀態。
這代表大量灌溉以及地下水位上升是這種新型液化災害的成因。
Bradley博士表示:「通常人為造成的災害比其他自然災害更容易防範。基於樹木作物跟稻田混種的地區更為強韌,我們提議多採用混種法可以降低未來大型山崩造成的災害。」
為了補足衛星影像的繪製結果,印尼國立亞齊大學海嘯與災害防治研究中心的Ella
Meilianda博士主持的國際團隊,實地觀察了山崩以及當地的灌溉系統和實施情形。
加州大學聖塔芭芭拉分校布倫環境科學學院的Thomas
Dunne教授並未參與這項研究。他說:「研究呈現出透過實地研究而對地表活動機制具有深刻瞭解的地球科學家,可以運用快速發展的各式遙測工具來分析致災過程。研究中以地景為尺度的調查方法,可以運用在其他地方進行系統性的災害評估與防範――這是在發展迅速但地體結構可能不甚穩定的地區,人們首度提出大型設施的建案時經常忽略的作業。」
研究團隊下一步的研究計畫是評估巴路地震發生時,當地土地利用方式對結果造成了什麼樣的影響。
Rice irrigation
intensified landslides in the deadliest earthquake of 2018
Irrigation significantly exacerbated the
earthquake-triggered landslides in Palu, on the Indonesian island of Sulawesi,
in 2018, according to an international study led by Nanyang Technological
University, Singapore (NTU Singapore) scientists.
The 7.5 magnitude earthquake struck the Indonesian
city on 28 September 2018, taking the lives of over 4,300 people, making it the
deadliest earthquake in the world that year.
Writing in Nature
Geoscience, researchers from NTU Singapore’s Earth Observatory of Singapore
(EOS) and the Asian School of the Environment (ASE), together with
collaborators from institutions in Indonesia, the United States, the United
Kingdom, China and Australia, reveal that the landslides in Indonesia’s Palu
Valley resulted from widespread liquefaction in areas that were heavily
irrigated for rice cultivation.
A century-old aqueduct, constructed to bring enough
water into the Palu Valley to irrigate rice, artificially raised the water
table to almost ground level. This elevation increased the potential for
liquefaction – a situation where buried sediment becomes fluid-like due to
strong seismic ground-shaking.
The combination of this fluid-like sediment and the
slope of the valley floor exacerbated the catastrophe, creating wide lateral
spreading of water, landslides, and debris, which swept through the villages.
This deadly cocktail marked Indonesia´s deadliest
earthquake since Yogyakarta in 2006.
“This event is a wake-up call for any area where
active faults and irrigation coincide,” said Dr Kyle Bradley, a principal
investigator at NTU’s EOS who led the research.
“We need to improve the awareness and understanding
of liquefaction-related landslides and pay closer attention to places where
irrigation has artificially raised the water table, said Dr Bradley, who is
also a lecturer at NTU’s ASE.
The research highlights the urgency for Southeast
Asian nation-states to review locations with intensive rice farming activities
which lie among active faults.
Dr Bradley said, “This is of particular concern in
Southeast Asia as the pace of development is often faster than the return time
of large earthquakes – the average time period between one earthquake and the
next. Most other similarly irrigated areas have not yet been tested by extreme
ground shaking, and some of those areas could also pose a major hazard.”
Research used
historic and current satellite data
By analysing satellite images taken before and after
the earthquake to identify areas affected by landslides, NTU researchers
discovered that irrigated paddies and fields were strongly affected, while
areas planted with trees were more stable.
This suggested that heavy irrigation and a raised
water table were responsible for creating a new liquefaction hazard.
“Hazards that are created by humans can often be more
readily moderated than other natural hazards. Based on the relative resiliency
of areas planted with mixed tree crops and irrigated fields, we propose that
more intermixed planting could decrease the hazard of large landslides in the
future,” said Dr Bradley.
The satellite image mapping was complemented by field
observations of the landslides and of the local irrigation system and
practices, produced by an international team of scientists led by Dr Ella
Meilianda of the Tsunami and Disaster Mitigation Research Center at Syiah Kuala
University in Banda Aceh.
Professor Thomas Dunne of the Bren School of
Environmental Science and Management at the University of California, Santa
Barbara, who was not affiliated with the study, said “The study has demonstrated
how Earth scientists with strong field-based understanding of land surface
mechanics can use the rapidly growing toolbox of remote sensing to analyse
dangerous processes. The landscape-scale survey approach could be applied
elsewhere for systematic assessment and avoidance of dangers that are often
overlooked when large infrastructure is first proposed in rapidly developing,
but potentially unstable terrains.”
The research team plans to continue their study by
assessing the effects of local land use on outcomes during the Palu earthquake.
原始論文:Kyle Bradley, Rishav Mallick, Harisma
Andikagumi, Judith Hubbard, Ella Meilianda, Adam Switzer, Nairong Du, Gilles
Brocard, Dedy Alfian, Benazir Benazir, Guangcai Feng, Sang-Ho Yun, Jedrzej
Majewski, Shengji Wei, Emma M. Hill. Earthquake-triggered 2018 Palu Valley landslides
enabled by wet rice cultivation. Nature Geoscience,
2019; DOI: 10.1038/s41561-019-0444-1
引用自:Nanyang Technological University. "Rice
irrigation intensified landslides in the deadliest earthquake of 2018, finds
NTU study"
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